1. Field of the Invention
The present invention relates to a method and device for measuring the waveform of an optical signal.
2. Description of the Related Art
In the field of optical fiber communication, the modulation rate of a signal continues to increase year after year. Recently, it has been seriously examined to use a signal speed of 100 Gb/s or more much higher than an electrical band by applying a time division multiplexing technique to multiplex an RZ signal of about 10 Gb/s. In researching and developing a technique related to such an ultrahigh-speed signal, a highly stable waveform measuring device having a time resolution of picoseconds to subpicoseconds is indispensable. In particular, the observation of an eye pattern as an overlaid display of signals is important from the viewpoint of application to communication.
As a device for measuring the eye pattern, a sampling oscilloscope is known. The sampling oscilloscope is a device using a signal sampling method to extract instantaneous voltage components, i.e., samples, of a periodic input signal at its sequentially different portions and to regenerate a high-frequency signal from the extracted many samples in a low-frequency region.
A time resolution in the sampling is uniquely determined by the pulse width of a trigger. At present, the band of a maximum-performance electrical sampling measurement device is limited by an electrical band, and it is about 50 GHz. Accordingly, the time resolution is about 20 ps at most.
Usually, in the case of measuring the eye pattern of an optical signal, the optical signal is once converted into an electrical signal by an opto/electrical converter, and the eye pattern of this electrical signal is next measured. Accordingly, even though an opto/electrical converter having a wide band greater than 50 GHz is used, the eye pattern of an optical signal having a width smaller than 20 ps cannot be measured.
As shown in FIG. 1, the terms of xe2x80x9csamplingxe2x80x9d means obtaining xe2x80x9cANDxe2x80x9d of an input signal and a trigger. In the case of FIG. 1, a pulse pattern generator (PPG) 4 generates a modulating signal according to a clock from an oscillator 2, and an LN (lithium niobate) optical modulator 6 modulates CW light (continuous-wave light) from a laser diode (LD) 8 according to the above modulating signal. An optical signal obtained by this modulation is transmitted by an optical fiber 10, and the transmitted optical signal is converted into electrical data by an O/E converter (opto/electrical converter) 12. Then, an electrical sampling circuit 14 measures the waveform of the input electrical data from the O/E converter 12 by using the clock from the oscillator 2 as a trigger.
As the O/E converter 12, an optical receiver having a band of about 0.60 GHz has already been put to practical use, and as the electrical sampling circuit 14, a device having a band of about 50 GHz has already been put to practical use. Accordingly, a time resolution of about 20 ps can be realized.
By obtaining the electrical xe2x80x9cANDxe2x80x9d as mentioned above, the eye pattern of a certain level high-speed signal can be stably measured. However, it is difficult to apply the electrical xe2x80x9cANDxe2x80x9d to the measurement of the eye pattern of a higher-speed signal of 100 Gb/s or more requiring a higher time resolution.
As a method for remarkably improving the time resolution, an optical sampling method is known, in which short pulse light of the order of picoseconds is used as the above-mentioned trigger, and an input optical signal and this optical trigger are input into a nonlinear medium to optically examine the cross correlation therebetween. To realize a high time resolution in the optical sampling method, ultrashort pulses of light with less phase noise are required as the optical trigger, and a nonlinear medium having ultrahigh-speed characteristics and ultrawide-band characteristics is indispensable as an AND circuit.
In the optical sampling method, the time resolution is determined by the pulse width and jitter of the optical trigger and by the response speed and group velocity dispersion of the nonlinear medium. It has been reported to use a nonlinear medium having sufficiently high-speed response characteristics and small group velocity dispersion, thereby effecting optical sampling with a time resolution of the order of picoseconds.
For example, optical sampling can be performed by using KTP having high-speed response characteristics of the order of subpicoseconds and a group delay difference of the order of subpicoseconds (interaction length: several millimeters) as the nonlinear medium and by adopting a method (SFG) of generating light corresponding to a sum frequency of an optical signal whose waveform is to be measured (which signal will be hereinafter referred to as xe2x80x9csubject optical signalxe2x80x9d) and an optical trigger. The time resolution is limited by the pulse width of the optical trigger, and it is about 8 ps with an S/N ratio of 22 dB. Accordingly, the waveform of an optical signal of 25 Gb/s can be measured. In general, an inorganic nonlinear medium such as KTP has a problem that the conversion efficiency is low.
As an improvement in the conversion efficiency, a method of using a nonlinear organic crystal as the nonlinear medium has been reported in IEE Electronics Letters, vol. 32, issue 24, Nov. 21, 1996, pp. 2256-2258. In this method, an organic crystal (AANP) having a high conversion efficiency about 10 times or more that of an inorganic nonlinear medium is used to improve the conversion efficiency, and an optical trigger of 0.4 ps is used to achieve a time resolution of 0.9 ps.
As another method for improving the conversion efficiency, a method of utilizing four-wave mixing (FWM) generated in a semiconductor optical amplifier (SOA) is known (IEEE Photonics Technology Letters, vol. 11, no. 11, 1999, pp. 1402-1404). In this method, a time resolution of 1.7 ps has been achieved. However, there is a future problem of whether or not accurate measurement can be made on an optical signal having an arbitrary pattern, because a pattern effect due to a carrier density modulation effect is exhibited in a SOA. Other methods of utilizing FWM generated in an optical fiber have been reported (IEE Electronics Letters, vol. 27, issue 16, Aug. 1, 1991, pp. 1440-1441 and J. Li, et al., xe2x80x9c300 Gbit/s eye-diagram measurement by optical sampling using fiber based parametric amplificationxe2x80x9d, Optical Fiber Communication Conference, Mar. 17-22, 2001, Anaheim, Calif.). In the latter report, the measurement of an eye pattern corresponding to 300 Gb/s is made.
The reason why the time resolution is limited by a group velocity will now be described with reference to FIG. 2. As shown in FIG. 2, group delays xcfx84sig and xcfx84tri per unit length of an optical fiber are produced in the optical signal and the optical trigger, respectively, by the group velocity. In general, xcfx84sigxe2x89xa0xcfx84tri because the group velocity differs according to wavelength, so that a relative temporal difference (walk-off) is induced between the optical signal and the optical trigger in the optical fiber.
To efficiently generate FWM, the wavelength xcextri of the optical trigger is generally set equal to the zero-dispersion wavelength xcex0 of the optical fiber. Accordingly, when the wavelength xcexsig of the optical signal is substantially equal to the zero-dispersion wavelength xcex0, the optical signal and the optical trigger cannot be separated from each other, and the waveform of the optical signal cannot therefore be observed near the zero-dispersion wavelength xcex0.
As means for basically eliminating the limitation to the time resolution by the group velocity, there has been proposed a method of using cross-phase modulation (XPM) generated in a nonlinear optical loop mirror (NOLM) (IEE Electronics Letters, vol. 27, issue 3, Jan. 31, 1991, pp. 204-205). The principle of sampling an optical signal by using a NOLM will now be described with reference to FIG. 3.
The NOLM has a first optical coupler including first and second optical paths directionally coupled to each other, a loop optical path formed of a nonlinear optical medium for connecting the first and second optical paths, and a second optical coupler including a third optical path directionally coupled to the loop optical path. The optical signal is branched into two components by the first optical coupler. One of the two components propagates clockwise in the loop optical path, and the other component propagates counterclockwise in the loop optical path. In the case that no optical trigger pulses are present, these two components are returned to the first optical coupler and interfere with each other. Then, resultant light is output from the same port (the first optical path) as the port from which the optical signal has been supplied.
When optical trigger pulses are introduced through the second optical coupler to the loop optical path and propagate with one of the two components of the optical signal in the loop optical path, this component undergoes phase modulation xc3x8=2xcex3PL by the third-order nonlinear optical effect of an optical fiber.
In the phase modulation xc3x8=2xcex3PL, xcex3 is the nonlinear coefficient of the optical fiber, P is the peak power of the optical trigger pulses, and L is the length of the optical fiber (loop optical path). When the component of the optical signal undergone the phase modulation interferes with the other component of the optical signal not undergone the phase modulation, resultant light by this interference is output from the other port (the second optical path) different from the port from which the optical signal has been supplied. That is, only the sampled component of the optical signal is output from the second optical path. The sampled component is output most efficiently under the condition where xc3x8=xcfx80, thereby maximizing an optical signal-to-noise ratio (OSNR).
In the case of using FWM, it is preferable to strictly equalize the center wavelength xcextri of the optical trigger to the zero-dispersion wavelength xcex0 of the optical fiber, in order to achieve the phase-matching condition. In contrast, in the method using XPM, the wavelengths of the subject optical signal and the optical trigger can be set to arbitrary wavelengths. However, to substantially suppress the walk-off between the subject optical signal and the optical trigger, it is preferable to symmetrically allocate the wavelength xcexsig of the optical signal and the wavelength xcextri of the optical trigger with respect to the zero-dispersion wavelength xcex0 of the optical fiber so that the group delays become equal.
In this case, however, there is a limit to the wavelength range of the subject optical signal, and a high time resolution of the order of subpicoseconds cannot be achieved. More specifically, in the case that the center wavelength xcexsig of the subject optical signal is near the zero-dispersion wavelength xcex0, the sampling light and the subject optical signal cannot be separated from each other in the above-mentioned symmetrical wavelength allocation, so that the walk-off is generated to limit the time resolution. As a result, a wavelength band where the sampling waveform cannot be accurately obtained is present near the zero-dispersion wavelength xcex0.
Further, the smaller the pulse width of the optical trigger, the more remarkable the influence of the group velocity dispersion, so that pulse broadening becomes unavoidable to limit the time resolution. While a time resolution of 7 ps has been reported in the last cited literature, a high time resolution of the order of subpicoseconds is difficult to achieve by the same configuration.
Further, there is an actual problem of fluctuations of the zero-dispersion wavelength of the optical fiber along the fiber length. In general, the fluctuations are periodical and range on the order of several nanometers. As the result of the fluctuations, the graph of the group time delay shown in FIG. 2 or 3 is shifted to the right or the left. Accordingly, in the case of using a long optical fiber as the loop optical path, the group time delay is locally shifted to cause a degradation in the time resolution.
It is therefore an object of the present invention to provide a method and device for measuring the waveform of an optical signal which can faithfully observe the waveform of the optical signal with a high time resolution.
In accordance with an aspect of the present invention, there is provided a method of measuring the waveform of an optical signal. In this method, a nonlinear optical loop mirror is provided, which comprises a first optical coupler including first and second optical paths directionally coupled to each other, a loop optical path formed of a nonlinear optical medium for connecting said first and second optical paths, and a second optical coupler including a third optical path directionally coupled to said loop optical path. An optical signal whose waveform is to be measured is supplied into said nonlinear optical loop mirror from said first optical path. An optical trigger having a predetermined pulse width is supplied into said nonlinear optical loop mirror from said third optical path. Information on the waveform of said optical signal is obtained according to light output from said second optical path. The predetermined pulse width is set according to a required measurement accuracy.
According to this method, the pulse width of the optical trigger is set according to the required measurement accuracy, so that the waveform of the subject optical signal can be faithfully observed with a high time resolution.
In accordance with another aspect of the present invention, there is provided a device for measuring the waveform of an optical signal. This device comprises a nonlinear optical loop mirror comprising a first optical coupler including first and second optical paths directionally coupled to each other, a loop optical path formed of a nonlinear optical medium for connecting said first and second optical paths, and a second optical coupler including a third optical path directionally coupled to said loop optical path; means for supplying said optical signal whose waveform is to be measured into said nonlinear optical loop mirror from said first optical path; means for supplying an optical trigger having a predetermined pulse width into said nonlinear optical loop mirror from said third optical path; means for obtaining information on the waveform of said optical signal according to light output from said second optical path; and means for setting said predetermined pulse width according to a required measurement accuracy.
In accordance with a further aspect of the present invention, there is provided a system comprising an optical fiber transmission line for transmitting an optical signal; and a waveform measuring device for receiving said optical signal transmitted by said optical fiber transmission line. The device according to the present invention may be used as said waveform measuring device.
In accordance with a still further aspect of the present invention, there is provided a system an optical fiber transmission line for transmitting an optical signal; a device provided along said optical fiber transmission line for processing said optical signal; and a waveform measuring device provided immediately downstream of said device for processing said optical signal. The device according to the present invention may be used as said waveform measuring device.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.